This invention relates to a method for making diaryl carbonate using a water resistant catalyst to effect the direct carbonylation of a phenolic compound (the “phenolic precursor”) to a diaryl carbonate. More particularly, the method relates to increasing the turnover number (TON) of the catalyst employed in the carbonylation reaction by making it less susceptible to degradation by the water generated as a by-product during the carbonylation reaction. The turnover number (TON) is a measure of the efficiency of the carbonylation catalyst. TON may be defined as the number of moles of a product diaryl carbonate produced per mole of catalyst employed.
Water removal techniques in phenol carbonylation reactions have been the subject of intense research in an effort to increase the useful life and TON of water-sensitive carbonylation catalysts. Water tends to poison Group 9 metal carbonylation catalysts typically employed during the carbonylation of phenolic compounds to diaryl carbonates, thereby increasing the overall cost of production.
Providing a means for water removal in carbonylation reactions has been shown to provide increased catalyst turnover numbers. In some instances carbonylation catalyst turnover numbers as high as about 6000 have been achieved when a means for water removal has been provided. Efficient water removal is expected to provide a manifold increase in the useful life of the carbonylation catalyst.
Current methods of overcoming the effects of water generated during the production of diaryl carbonates include water removal by molecular sieves, and water entrainment out of the reaction mixture by evaporation. Water removal using molecular sieves may yield high TONs but the use of molecular sieves is marked by disadvantages such as cost, fines production, handling and regeneration difficulties, and the like.
The present invention provides an alternate approach to increasing the carbonylation catalyst turnover number by making the carbonylation catalyst water resistant.
Since overcoming the negative effects of water produced in the carbonylation reaction using molecular sieves, evaporative removal of water from the carbonylation reaction mixture, and like methods entails added costs (e.g. cost of molecular sieves or the increased investment costs required to develop an evaporation column), it would be desirable to provide a method for making diaryl carbonates which benefited from the use of a water resistant carbonylation catalyst. The present invention provides a method for the carbonylation of phenolic compounds which employs one or more chemical agents which improve the stability of the noble metal carbonylation catalysts typically employed during the carbonylation of phenolic compounds to diaryl carbonates. Due to this enhanced resistance to the deleterious effects of water, the method of the present invention achieves more efficient use of the noble metal carbonylation catalysts employed. This and other purposes of the present invention are disclosed in detail herein.